Recently, a solar power generation as one of a clean energy is increasingly focused on and a crystalline Si solar cell is employed mainly, while a CIGS (Cu—In—Ga—Se quaternary alloy)-based solar cell is also focused on especially because of its high conversion efficiency in view of supply and cost issues.
A CIGS solar cell has a basal structure consisting of a Mo electrode layer serving as a back side electrode formed on a soda lime glass substrate, a Cu—In—Ga—Se quaternary alloy film serving as a light-absorbing layer formed on this Mo electrode layer, a buffer layer comprising ZnS, CdS and the like formed on the light-absorbing layer formed on this Cu—In—Ga—Se quaternary alloy film and a transparent electrode formed on this buffer layer.
While a vapor deposition method is known as a method for forming a light-absorbing layer consisting of a Cu—In—Ga—Se quaternary alloy film, a formation by a sputtering method is proposed for the purpose of obtaining a uniform film over a larger area. The sputtering method is a method in which an In target is employed first for forming an In thin layer by sputtering, and then on this In thin layer a Cu—Ga alloy thin layer is formed by sputtering using a Cu—Ga alloy sputtering target, and the resultant laminated thin layers consisting of the In thin layer and the Cu—Ga alloy thin layer is subjected to a heat treatment in a Se atmosphere to form a Cu—In—Ga—Se quaternary alloy thin layer. A high quality Cu—Ga alloy sputtering target is desirable since the quality of this Cu—In—Ga—Se quaternary alloy film made by the sputtering method depends greatly on the quality of the Cu—Ga alloy sputtering target.
As a method for producing a Cu—Ga alloy sputtering target, a melting method and a powder sintering method are known. The melting method involves a problem that the Cu—Ga alloy of a CIGS solar cell-based composition which was conducted by melting/casting is brittle and readily broken. On the other hand, the powder sintering method is regarded as a hopeful method for producing a sputtering target which has a uniform composition.
As a powder sintering method, a method is described in Patent Document 1 in which a Cu—Ga alloy powder having a high Ga-content is mixed with a pure Cu or mixed with a Cu—Ga alloy powder having a low Ga-content to produce a sputtering target by hot press.